1. Field of the Invention
The present invention relates to a control circuit for a thermostatic oven in an oven controlled crystal oscillator, and more particularly to a control circuit capable of further improving stability of the oscillation frequency of an oven controlled crystal oscillator by performing temperature-based compensation for the oscillation frequency with high accuracy.
2. Description of the Related Art
An oven controlled crystal oscillator (OCXO) has a crystal resonator disposed inside a case whose temperature is maintained constant, and has a stable frequency characteristic which is not easily influenced by an environmental temperature.
A control circuit for a thermostatic oven to be used in an oven controlled crystal oscillator has a heater which heats the thermostatic oven, and a thermistor whose resistance value changes according to the temperature, and controls the temperature in the thermostatic oven to be constant by adjusting a voltage to be applied to the heater according to the ambient temperature of the thermostatic oven.
To perform temperature-based compensation for the oscillation frequency of a crystal oscillator, the voltage is adjusted based on a temperature detected by a temperature sensor provided outside the oscillator to compensate for the oscillation frequency thereof.
Related arts relating to an oven controlled crystal oscillator are disclosed in Japanese Utility Model Application Laid-Open No. S64-7320 (Patent Document 1) and Japanese Patent Application Laid-Open No. H3-126303 (Patent Document 2).
The Patent Document 1 describes a thermostatic oven temperature control circuit which is configured to have a nichrome wire heater as a heating element provided in a temperature setting bridge and set the temperature according to a change in the resistance value of the heating element, and eliminates the need for a temperature sensor such as a thermistor.
The Patent Document 2 describes a temperature compensated crystal oscillator in which a transistor serving as an oscillation active element is used as a temperature sensor and a frequency control element, and which has a circuit configuration to facilitate integration.
Patent Document 1: Japanese Utility Model Application Laid-Open No. S64-7320
Patent Document 2: Japanese Patent Application Laid-Open No. H3-126303
Because the conventional crystal oscillator compensates for the oscillation frequency based on a temperature detected by the temperature sensor provided outside the oscillator, however, the temperature to be detected slightly differs from the temperature inside the oscillator where the crystal resonator is actually disposed, disabling more accurate temperature compensation.
In the conventional oven controlled crystal oscillator, the thermistor is used only for controlling the temperature of the thermostatic oven, and a signal according to the ambient temperature of the thermostatic oven which is to be detected by the thermistor is not used in control or the like of the oscillation frequency.
Further, the stability of the frequency of the conventional oven controlled crystal oscillator is 10−9 to 10−10 or so, and is not sufficient in a case of demanding a higher stability.